1. Technical Field
The present invention relates to a method for manufacturing a surface acoustic wave element, especially a method for manufacturing a surface acoustic wave element including a step for adjusting the resonant frequency of a surface acoustic wave element formed on a piezoelectric substrate to a target resonant frequency, as well as a surface acoustic wave element that is manufactured by the same manufacturing method.
2. Related Art
With the development of communication technologies in recent years, the frequency of surface acoustic wave (SAW) devices such as SAW resonators, SAW filters, etc. that employ a surface acoustic wave element has become higher. The resonant frequency of a surface acoustic wave element depends on the pitch of a comb-shaped electrode that configures an interdigital transducer (IDT). The comb-shaped electrode has been more miniaturized with the higher frequency of SAW devices. Therefore, the formation of electrodes for a high-frequency surface acoustic wave element has been performed by microprocessing using a reduced projection exposure device (as described in a first related art example, which will be described later). However, the resonant frequency of surface acoustic wave elements cannot be made as accurate as required only by microprocessing electrodes using a reduced projection exposure device due to the influence of thickness variations of the conductive films configuring the electrodes, the fabrication error of the formed electrodes, etc. Hence, in the conventional technique, frequency is adjusted for each surface acoustic wave element so as to obtain a frequency with required accuracy. Further, in a second related art example, which will be described later, a method for adjusting resonant frequency by performing anodic oxidation of comb-shaped electrodes or reflectors while the surface acoustic wave elements are still formed on a wafer (piezoelectric substrate), avoiding the increase of electric resistance due to anodic oxidation of the electrodes performed after cutting the wafer into pieces of surface acoustic wave elements, is disclosed.
Japanese Unexamined Patent Publication No. 5-283970 is the first related art example.
Japanese Unexamined Patent Publication No. 6-164287 is the second related art example.
In the frequency adjustment method according to the second related art example (hereinafter referred to as the conventional frequency adjustment method), however, anodic oxidation is performed by setting an anodic oxidation voltage within the range of several dozen to several hundred volts in accordance with a desired frequency, as described in Paragraph No. 0014 of the second related art example. Therefore, it is difficult to achieve an accurate resonant frequency adjustment. Besides, in the conventional frequency adjustment method, frequency is adjusted with a single attempt of anodic oxidation performed based on an anodic oxidation voltage determined by calculating the amount of frequency adjustment. Therefore, the conventional frequency adjustment method causes a difference in frequency variation due to anodic oxidation, depending on variations among piezoelectric substrates (wafers) in electrode dimensions including the thickness of an electrode film (conductive film), the width of an IDT electrode finger (hereinafter simply referred to as an electrode, occasionally), the pitch of electrode fingers, etc. As a result, in the conventional frequency adjustment method, the variation in resonant frequency among piezoelectric substrates after anodic oxidation, that is, the variation among the average resonant frequencies of surface acoustic wave elements on each piezoelectric substrate becomes large. Therefore, the conventional frequency adjustment method requires further frequency adjustment of each surface acoustic wave element after performing anodic oxidation of electrodes.